The present invention relates to work holders, and more particularly to a swing clamp for holding a workpiece.
Powered work clamping devices or swing clamps are typically used on machine tools to hold a workpiece on a table or jib while a mechanical operation such as milling, drilling, or grinding is performed. Swing clamps may be hydraulically or pneumatically powered, and as schematically shown in FIG. 1 generally include a housing 1 defining an upper chamber 2 and a lower chamber 3, a piston 4 disposed in upper chamber 2 connected to a plunger 5 having a work-engaging head 6 thereon, and a piston 7 in lower chamber 3 connected to a rod 8 which is slidably received within plunger 5. Such swing clamps also typically include three solenoid actuated two position flow control valves for controlling the flow of hydraulic fluid to and from the clamp. As shown, valve 9 is normally opened and is connected via line 10 to port A at the upper end of chamber 2. A second normally closed valve 11 is connected via line 12 to port C at the upper end of chamber 3 and via line 13 to port B at the lower end of chamber 2. A third normally closed valve 14 is connected via line 15 to the lower end of chamber 3.
FIG. 1 also shows sequentially the steps for clamping and unclamping a workpiece 16 on a table 17. In its home position, head 6 is retracted within table 17 with valve 14 normally closed so that tank pressure is communicated to port D, valve 11 normally open so that tank pressure is communicated to ports B and C, and valve 9 is normally open applying working pressure to port A thus holding head 6 in a retracted position. To extend head 6, valves 9, 11 and 14 are all energized so that working pressure is communicated to ports B, C, and D and tank pressure is communicated to port A. To swing head 6, valve 14 is de-energized so that tank pressure is communicated to port D and piston 7 may be moved downwardly in chamber 3. A rotation mechanism (not shown) coacting between plunger 5 and rod 8 causes head 6 to swing as rod 8 and piston 7 are moved downwardly. To clamp workpiece 16, all three valves 9, 11, and 14 are de-energized so that work pressure is applied at port A and tank pressure is communicated to ports B, C and D. To unclamp workpiece 16, head 6 is first extended by energizing valves 9 and 11 so that work pressure is applied to port B to extend head 6. Head 6 is then rotated by energizing valves 9 and 14 and de-energizing valve 11 so that tank pressure is communicated to ports A, B and C, and work pressure is applied to port D so that piston 7 and rod 8 move upwardly to coact with plunger 5. Finally, to retract head 6 into its home position, valve 14 is energized and valves 9 and 11 are de-energized so that work pressure is felt at port A to move piston 4, plunger 5 and head 6 downwardly.
A swing clamp system such as that shown in FIG. 1 are relatively expensive since it requires three control valves (9, 11, and 14) for its operation. It is thus desireable to provide a swing clamp system that employs a single control valve so that the cost of such a control system can be minimized and the operation simplified.